1. Introduction
In topic 9d, an ecosystem was defined as a dynamic entity composed of a
biological community and its associated abiotic environment. Often the
dynamic interactions that occur within an ecosystem are numerous and
complex. Ecosystems are also always undergoing alterations to their
biotic and abiotic components. Some of these alterations begin first with
a change in the state of one component of the ecosystem which then
cascades and sometimes amplifies into other components because of
relationships.
In recent years, the impact of humans has caused a number of dramatic
changes to a variety of ecosystems found on the Earth. Humans use and
modify natural ecosystems through agriculture, forestry, recreation,
urbanization, and industry. The most obvious impact of humans on
ecosystems is the loss of biodiversity. The number of extinctions caused
by human domination of ecosystems has been steadily increasing since
the start of the Industrial Revolution. The frequency of species
extinctions is correlated to the size of human population on the Earth
which is directly related to resource consumption, land-use change, and
environmental degradation. Other human impacts to ecosystems include
species invasions to new habitats, changes to the abundance and
dominance of species in communities, modification of biogeochemical
cycles, modification of hydrologic cycling, pollution, and climatic
change.
Major Components of Ecosystems
Ecosystems are composed of a variety of abiotic and biotic components
that function in an interrelated fashion. Some of the more important
components are: soil, atmosphere, radiation from the Sun, water, and
living organisms.
Soils are much more complex than simple sediments. They contain a
mixture of weathered rock fragments, highly altered soil mineral
particles, organic matter, and living organisms. Soils provide nutrients,
water, a home, and a structural growing medium for organisms. The
vegetation found growing on top of a soil is closely linked to this
component of an ecosystem through nutrient cycling.
The atmosphere provides organisms found within ecosystems with
carbon dioxide for photosynthesis and oxygen for respiration. The

2. processes of evaporation, transpiration, and precipitation cycle water
between the atmosphere and the Earth's surface.
Solar radiation is used in ecosystems to heat the atmosphere and to
evaporate and transpire water into the atmosphere. Sunlight is also
necessary for photosynthesis. Photosynthesis provides the energy for
plant growth and metabolism, and the organic food for other forms of life.
Most living tissue is composed of a very high percentage of water, up to
and even exceeding 90%. The protoplasm of a very few cells can survive
if their water content drops below 10%, and most are killed if it is less
than 30-50%. Water is the medium by which mineral nutrients enter and
are translocated in plants. It is also necessary for the maintenance of leaf
turgidity and is required for photosynthetic chemical reactions. Plants and
animals receive their water from the Earth's surface and soil. The original
source of this water is precipitation from the atmosphere.
Ecosystems are composed of a variety of living organisms that can be
classified as producers, consumers, or decomposers. Producers or
autotrophs, are organisms that can manufacture the organic compounds
they use as sources of energy and nutrients. Most producers are green
plants that can manufacture their food through the process of
photosynthesis. Consumers or heterotrophs get their energy and
nutrients by feeding directly or indirectly on producers. We can
distinguish two main types of consumers. Herbivores are consumers that
eat plants for their energy and nutrients. Organisms that feed on
herbivores are called carnivores. Carnivores can also consume other
carnivores. Plants and animals supply organic matter to the soil system
through shed tissues and death. Consumer organisms that feed on this
organic matter, or detritus, are known as detritivores or decomposers.
The organic matter that is consumed by the detritivores is eventually
converted back into inorganic nutrients in the soil. These nutrients can
then be used by plants for the production of organic compounds.
The following graphical model describes the major ecosystem
components and their interrelationships (Figure 9j-1).

3. Figure 9j-1: Relationships within an ecosystem.
Energy and Matter Flow in Ecosystems
Many of the most important relationships between living organisms and
the environment are controlled ultimately by the amount of available
incoming energy received at the Earth's surface from the Sun. It is this
energy which helps to drive biotic systems. The Sun's energy allows
plants to convert inorganic chemicals into organic compounds.
Only a very small proportion of the sunlight received at the Earth's
surface is transformed into biochemical form. Several studies have been
carried out to determine this amount. A study of an Illinois cornfield
reported that 1.6% of the available solar radiation was photosythetically
utilized by the corn. Other data suggests that even the most efficient
ecosystems seldom incorporate more than 3% of the available solar
insolation. Most ecosystems fix less than 1% of the sunlight available for
photosynthesis.
Living organisms can use energy in basically two forms: radiant or
fixed. Radiant energy exists in the form of electromagnetic energy,
such as light. Fixed energy is the potential chemical energy found in
organic substances. This energy can be released through respiration.
Organisms that can take energy from inorganic sources and fix it into
energy rich organic molecules are called autotrophs. If this energy
comes from light then these organisms are called photosynthetic

4. autotrophs. In most ecosystems plants are the dominant photosynthetic
autotroph.
Organisms that require fixed energy found in organic molecules for their
survival are called heterotrophs. Heterotrophs who obtain their energy
from living organisms are called consumers. Consumers can be of two
basic types: Consumer and decomposers. Consumers that consume plants
are know as herbivores. Carnivores are consumers who eat herbivores
or other carnivores. Decomposers or detritivores are heterotrophs that
obtain their energy either from dead organisms or from organic
compounds dispersed in the environment.
Once fixed by plants, organic energy can move within the ecosystem
through the consumption of living or dead organic matter. Upon
decomposition the chemicals that were once organized into organic
compounds are returned to their inorganic form and can be taken up by
plants once again. Organic energy can also move from one ecosystem to
another by a variety of processes. These processes include: animal
migration, animal harvesting, plant harvesting, plant dispersal of seeds,
leaching, and erosion. The following diagram models the various inputs
and outputs of energy and matter in a typical ecosystem (Figure 9j-2).
Figure 9j-2: Inputs and outputs of energy and matter in a typical
ecosystem.